In digital color image input apparatuses (such as digital scanners, digital still cameras, and the like), tristimulus color information (R, G, B) is obtained via a solid-state image sensing element (CCD) that serves as a color separation system. The tristimulus color information, which is obtained in a form of analog signals, is then converted to digital signals, which are used as input signals that represent input color image data (color information). Segmentation is carried out so that display or output is carried out most suitably according to the signals obtained via the image input apparatus. The segmentation partitions a read document image into regions of equivalent properties so that each region can be processed with image process most suitable thereto. This makes it possible to reproduce a good-quality image.
In general, the segmentation of a document image includes discriminating a text region, a halftone region (halftone area) and photo region (in another words, continuous tone region (contone region), which is occasionally expressed as other region) in the document image to read, so that quality improvement process can be switched over for the respective regions determined. This attains higher reproduction quality of the image.
Furthermore, the halftone regions (image) have halftone varied from low frequencies to high frequencies, such as 65 line/inch, 85 line/inch, 100 line/inch, 120 line/inch, 133 line/inch, 150 line/inch, 175 line/inch, 200 line/inch, and the like. Therefore, various methods have been proposed for determining halftone frequencies so as to perform suitable process according to the determination.
For example, Japanese Unexamined Patent Publications, Tokukai, No. 2004-102551 (published on Apr. 2, 2004; hereinafter “Patent Document 1”), discloses a method for determining a halftone frequency based on a number of changeover (i.e., transition number) of the binary values of binary data of an input image. Moreover, Japanese Unexamined Patent Publication, Tokukai, No. 2001-218046 (published on Aug. 10, 2001) (hereinafter, just referred to as Patent Document 2) discloses a method in which a similar peak is determined from a degree of similarity between (a) a current block and (b) a block located within a region which is distanced from the current block by a given number of pixels, and if the region is a halftone region, a halftone frequency is determined (i.e., found out) based on a peak nearest to a center of the halftone region.
According to Patent Document 1, whether the halftone is composite halftone or single-color halftone is not taken into consideration when the binarization and the calculating of the transition number. Moreover, according to Patent Document 2, like Patent Document 1, whether the halftone is composite halftone or single-color halftone is not taken into consideration. Therefore, it is difficult both in Patent Documents 1 and 2 to accurately determine the halftone frequency with respect to the composite halftone region.
Moreover, Japanese Patent No. 3093235 (issued on Oct. 3, 2000), and Japanese Unexamined Patent Publication No. 2002-77623 (published on Mar. 15, 2002) discloses a method in which halftone frequency determination is performed based on a number of peak pixels, which is a number of peak pixels counted in a predetermined number of blocks where the peak pixels are found using a mask of M pixels×N pixels (where M and N are integers predetermined experimentally).
According to the above method, the halftone frequency determination is performed based on the number of peak pixels in the predetermined number of blocks. However, a composite halftone and a single-color halftone of same halftone frequency give largely different numbers of peak pixels, where the composite halftone is a halftone consisting of at least two of cyan (hereinafter, C), magenta (hereinafter, M), yellow (hereinafter, Y), and black (hereinafter K), and the single-color halftone is a halftone consisting of one of CMYK. In other words, it is difficult to distinguish the composite halftone and the single-color halftone having similar numbers of peak pixels but different halftone frequencies. For example, it is difficult to distinguish a 133-line/inch composite halftone and 175-line/inch single-color halftone, which have similar numbers of peak pixels. Therefore, it is impossible to extract a number of peak pixels of a particular color component.